Various types of compressed gases are commonly transported in long, narrow cylinders, or “tubes,” mounted directly on a tractor trailer chassis or in a “module,” or a box frame containing the cylinders that is loaded onto a flat bed trailer. These truck-borne compressed gas cylinders are often required by law to be fitted with relief devices designed to relieve pressure from the compressed gas cylinders in the event of cylinder over-pressurization or fire. These relief devices are typically attached to each end of a cylinder and take two major forms: cluster-type relief devices (typically for hazardous gases) and angle-type relief devices (typically for non-hazardous gases). The relief devices typically protrude from the compressed gas cylinders and are subject to shearing forces. These relief devices provide protection for over pressurization, however, due to their physical structure, are prone to inadvertent damage such as having the relief device sheared off of the compressed gas cylinder. When such a shearing of a valve or relief device occurs, the compressed gas escapes through an uncontrolled opening in the compressed gas cylinder to the atmosphere.
In U.S. Pat. No. 5,832,947, entitled “Gas Shut-Off and Pressure Relief Valve for a High Pressure Gas Vessel,” issued to Andrew Niemczyk, a pressure relief valve is disclosed. The disclosed pressure relief valve has a threaded body that engages the side port in fluid communication with the gas passage. The side port has a radially extending shoulder that seats an annular sealing ring. A rupture disc, made from brass, is pressed by the threaded body against the sealing ring. During operation, the gas shut-off and pressure relief valve extends away from the compressed gas cylinder. The relief valve provides protection from an unintentional over pressurization; however, the protrusion of the relief valve increases the probability of an inadvertent shear of the relief valve.
In U.S. Pat. No. 4,269,214, entitled “Safety Pressure Relief Device,” issued to Calvin. C. Forsythe, et al., a safety pressure relief device is disclosed. The disclosed safety pressure relief device has a casing with a threaded connection and a concentric axial bore there through. The open end of the casing engages a concave-convex rupture disc which is ruptured with a knife means including a plurality of spaced cutting teeth. An annular outlet ring is connected to the casing by a continuous heli-arc weld. The weld also connects the rupture disc and the knife means to the casing. Again, protection is provided for an over pressurization. However, the probability of an inadvertent shear of the relief device is increased.
FIG. 1 illustrates another prior art relief device and its attachment to a compressed gas cylinder. The relief device 20 does not attach directly to the cylinder 10. Rather, the relief device 20 screws into a “bullplug” 30, which itself screws into the cylinder 10. Thus, the bullplug 30 has two sets of threads: a set of male straight threads 32 that engage a reciprocal female set 12 in the cylinder; and a set of female pipe threads 34 that engage a reciprocal set of male pipe threads 24 on the relief device 20. Moreover, having the relief device 20 screw into the bullplug 30, which in turn screws into the cylinder 10, means that the relief device 20 protrudes farther from the cylinder 10 than it would if the relief device 20 screwed directly into the cylinder 10. This greater protrusion presents a greater opportunity for a shear of the relief device 20 from the cylinder 10 in the event of an accident, resulting in uncontrolled compressed gas leakage.
Therefore, a need exists for a shear protection device that couples directly to its associated compressed gas tank utilizing a valve or relief device, thereby lessening the likelihood of a valve or relief device shear—and the associated uncontrolled leakage—in the event of an accident.
Accordingly, it is an object of the present invention to provide a shear protection device for compressed gas cylinders that attaches directly to its cylinder. It is a further object of this invention to provide a shear protection device that attaches directly to relief devices for compressed gas cylinders that reduces or eliminates the likelihood of the uncontrolled compressed gas leakage due to the valve or relief device shearing away from the main body in the event of an accident.